Category: "Weather and Climate"

Capturing Falling Snow in a Cold Fluid

February 15th, 2021

Snow is usually imaged in air, the single crystals laying flat on some substrate such as glass. The method is relatively simple, but one must work fast to image the crystal before it appreciably sublimates. Sublimation first rounds out the sharp edges and then causes the crystal to shrink. Generally, this sublimation happens because the photographer is radiating too much heat to the crystal. Conversely, particularly in very cold conditions, the photographer’s breath may deposit fog near the crystal, causing the crystal to grow.

Such issues vanish if one instead captures the snow in a cold fluid before taking the image. To work, this fluid should not dissolve the ice, be less dense than ice, be fluid enough to completely spread over the crystal surface, and be transparent. Other than preserving the crystal, the method has several other advantages. For example, in his laboratory experiments in Hokkaido, Japan, Tsuneya Takahashi lets the crystal fall into a cold suspension of two transparent, cold silicone oils. He sets it up so one fluid is denser than ice, one is lighter than ice, so the crystal falls through the light oil and rests on the (transparent) interface between the two fluids.

This method sounds complicated, so why use it? One, as the oils are immiscible with water, they block water molecules from arriving or leaving the crystal surfaces, so the ice crystal shape is preserved precisely for as long as the fluid is below 32 F (0 C). Two, after imaging the crystal, the fluid is warmed above melting such that the crystal melts into a spherical drop from which he can easily measure the volume and thus infer the mass of the original crystal. A third advantage, more difficult to exploit yet sometimes used, is that he can get top and side views of the same crystal. Other researchers in Japan have also used silicone oils to capture ice crystals in the lab, as well as naturally falling crystals, mainly for the first and third reason. They use just one oil type, a lighter oil. Charles Knight at NCAR in Boulder, Colorado had a fourth reason for using a cold fluid: better imaging. That is, one can image greater depth detail because light scattering off the surfaces is greatly reduced, particularly if the fluid is very clear and has an index of refraction close to that of ice. By reducing the scattering, one can see through surfaces to underlying surfaces. He would use gasoline or hexane fluid.

Capturing Falling Snow in a Cold Fluid

I don’t have a photomicrography setup to take detailed images of snow, and we rarely get snowfall with nice single crystals anyway, but I wondered how well the method might capture falling snowflakes. That is, could I at least see their rough shapes as they fell through the fluid?

Here, we typically get about one light snowfall per winter (2-3"). A relatively large snowfall happened this past weekend, depositing about seven inches. At first, the particles were small, probably highly rimed single crystals or small aggregates. Later, larger snow particles fell, and these particles were clearly snowflakes (i.e., aggregates). In preparation, the previous night I set out two covered wide-mount jars, one with water, the other with Coleman camping fuel (white gas), which has similar properties to gasoline. In the morning, the one with water had frozen, so I knew the other was also below 32 F.

Capturing Falling Snow in a Cold Fluid

Outdoors, I set the jar on top of my car, put a wooden chopstick in the jar both to focus on (my camera only has autofocus) and to provide a size reference, set a small LED light panel to the side for brighter illumination, and then opened the lid. The flakes fell into the fluid, and fell down to the bottom of the jar. They fell through the fluid slower than they fell through the air, but it was still too fast for me to see how well they were focused. Turned out that they were not very sharply focused, yet one can still see their general shape and fall orientation (below). In general, the flatter the flake, the more it tends to orient broadside to the fall direction.

Capturing Falling Snow in a Cold Fluid

Obvious improvements would be a better jar, such as one with a flat, smooth glass front, a better camera, and a thicker fluid to slow down the rate of fall.

Such improvements will have to wait at least until next winter.


A rare heliac arc? (plus six others)

February 5th, 2019

The day starts sunny and bright, but later you note a slight muting of the surrounding landscape. It is still bright enough, but you feel less heat bearing down. Apparently, a thin veil of high clouds has slowly and silently appeared above.

When this happens, please take a few seconds to look up and see what these clouds are up to. Do they have their crystals lined up for one or more arcs, or are they oriented randomly, giving a halo with perhaps a sun dog or two? To the trained eye, the chances are high that such a veil will give rewards, even when no crystals are present. But the crystals give so much more, particularly when they line up in various ways.

Wandering, somewhat exhausted, on the hillside above Index, WA last Monday, I got this very sense of a muted landscape. So, I found a break in the trees, looked up, and instantly felt recharged. A colorful circumzenithal arc had appeared, which by itself is always a treat. But I was also awed by several other arcs and a very diffuse 22-degree halo. Excitedly, I took photos and scrambled around trees and rock for a more complete view.

A rare heliac arc? (plus six others)

(As with all images here, click on the image to see a larger view.)

Due to the complex way the eye and brain discerns light and patterns versus the much simpler way of a camera, the patterns are much more distinct when viewed direct by eye. But the above composition has my attempt to compensate the original photo at left, with some contrast enhancement in the middle version, and markings on the right version.

The band of color at the intersection of CZA and SLA is from the oriented prism crystals of the circumzenithal arc (CZA). Their formation is relatively common I think, though a group of observers in Germany apparently finds their occurrence there to be only about 13 times per year ( What is most remarkable to me is not their colors, but the fact that formation of distinct colors requires that the crystals stay extremely level as they fall (deviations of only a few degrees would wash out the colors). My sketch below shows roughly what is going on here:

A rare heliac arc? (plus six others)

(Hold on for a moment, and I'll get to the rare heliac arc below.)

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Film frost grains and radiative cooling of the ground

December 10th, 2017

December 8th brought the first frost to the Seattle area. This doesn't mean that this is the first time this season that the ground reached 0 degrees C or lower. True, we had gotten snow in late November, though this by itself doesn't mean the ground reached 0 C because snow deposition differs from frost formation. The snow was below zero when it formed in the air, but for the rest of its existence, it could have been melting and the ground itself likely sped up the process by remaining slightly above 0 C. But in contrast with the snow, for frost to form, the ground itself must cool below 0 C.

When I used to put out some metal plates with recording thermocouples, I didn't see visible frost until about -5 C, but that was based on just a handful of measurements. Anyway, what this means is that we may have had a few mornings with some patches of ground (including anything connected to the ground) dipping slightly below zero but with no obvious frost appearing. Also keep in mind that "ground temperatures" reported at weather stations are 1.5-2.0 meters above the ground, and thus may be 5-10 degrees C warmer at night than some ground patches. Why? At night, the ground cools by radiating, and if the atmosphere is not radiating much down, then considerable cooling happens. This is why clear nights are the ones with the most frost or dew.

Anyway, here is one shot of some of this "first frost" on my car window.

Film frost grains and radiative cooling of the ground

The image shows patches of different texture. These regions differ in texture because they are tiny hoar-frost (i.e., vapor-grown) crystals of differing size or orientation. That is, they stick up differently in different patches. They stick up differently because they sprouted off of a thin layer of film-frost that had a different crystal orientation. So, the patchy look comes from the different grains in the film-frost. See some of my previous posts with diagrams about this phenomena (category: "film frost"). Here is one with particularly helpful diagrams.

-- JN

A Fogbow

October 25th, 2017

A fogbow, or cloudbow (fog is a type of cloud), is a special type of rainbow. It is just white, and so not as often photographed as the full-spectrum rainbow, but it can be exciting to see nevertheless.

A Fogbow

The reason the fogbow is white is because the water droplets in fog are much smaller than raindrops. Fog droplets may vary in diameter roughly between 1 and 20 millionths of a meter (i.e., 1-20 microns), whereas raindrops are typically 1-3 thousandths of a meter, or about 500 times larger. The wavelength of visible light is only about a half a micron, so the light rays inside a fog droplet are still fairly well defined, but there is simply not enough room to separate out the colors, to state things simply.

Upon approaching a fog in the morning, look towards, but above your shadow. About 50-60 degrees from the shadow of your head is where the fogbow will sit, just as it would for a rainbow. Evenings will work too. But midday, the angles 50-60 degrees above your shadow will have you looking at the ground, so you probably won't see a fogbow there. (If you are in an outdoor shower, you might see a rainbow though.)

The above photograph shows the fogbow I saw yesterday morning, about 8:30 am, biking into a nearby park.

-- JN

How clouds form snow

January 14th, 2017

To understand snow formation, one must know a little about clouds. 

Q: What is in a cloud?

A: Air, dust, vapor, droplets, and often, ice. 

Q: How much air? How much liquid water? How much ice?

A: The answers will probably surprise you. See my short 20-min presentation below. I gave this recently to the Bellingham, WA Snow School. (23 slides, but due to file-upload-size restrictions, I had to put them into three parts below, 10 slides, 6 slides, 7 slides.)

Snow, rain, and weather affect everybody, yet how many of us learned in school even the most basic facts about precipitation in school?

Q: Who first realized how ice grew in a cloud?

How clouds form snow

As described in my presentation, he realized this by observing frost on the ground. 

Q: Who first realized how Alfred's theory was intimately connected with rainfall? 

How clouds form snow

Tor discovered this by observing fog in a mountain forest, and like Alfred, applied some of his physics knowledge. 

In my presentation, I discussed Alfred Wegener, the roles of the different cloud components, and briefly how the ice, once formed, takes on its strange shapes: 


First 10 slides (with blue text added to account for the things I said during the talk):


Next 6 slides:


Last 7 slides:



Later, I will show specifically how the ice gets arranged into all these strange shapes. 

- JN

The new ice-crystal-growth apparatus

July 1st, 2014

After a few years in the making, our new device for growing single ice crystals in a well-controlled laboratory environment is nearly ready. We are just adding a few small accessory pieces to allow us to start testing. I was to describe the apparatus at the cloud-physics conference this month in Boston, and made a poster to present, but decided to opt out. But, having spent a few days making the poster, I present it below.

As with all images here, click on image to see large-scale view.

And below is the same, but in blog format.

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Halo in the sky? Uh, I don't see no halo...

April 20th, 2014

After a few days of fine bright spring weather, the barometer falls and a south wind begins to blow. High clouds, fragile and feathery, rise out of the west, the sky gradually becomes milky white, made opalescent by veils of cirro-stratus. The sun seems to shine through ground glass, its outline no longer sharp, but merging into its surroundings. There is a peculiar, uncertain light over the landscape; I 'feel' that there must be a halo round the sun!
And as a rule, I am right.

The quote, from Minneart* describes a common ice-related atmospheric apparition. It appears in skies all over the world far more often than the rainbow, yet few notice it. As a graduate student, I read about halos and often looked for one, but didn't notice it myself until someone else pointed it out. As a post-doc in Boulder, I was out walking with Charles Knight, and I mentioned my lack of success. He glanced up near the sun, pointed, and said “why there's one right now”.

What I had missed in my readings had been the fact that most halos are rather indistinct and often incomplete circles. Indeed, now when I point out the most common one (the 22-degree halo) to someone nearby, they often don't see it. But occasionally, it is sharp enough (and colored) to the extent that anyone will see it if they bother to look up and glance toward the sun. And often it occurs with other ice-crystal apparitions that are even more obvious.

Last fall, while perched high on a rock face, belaying my partner up**, I saw such a vivid display.

The bright spot is called a “sun dog”, “mock sun”, or “parhelia”. They, one on either side of the sun, usually appear together with the 22 degree halo. Indeed, the sun dogs very nearly mark the spots where that halo intersects another arc called the parahelic circle. Their cause: horizontally oriented, tabular ice crystals.

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The end of snow

March 1st, 2014

This recent front-page article caught my eye:

The writer is an avid skier-snowboarder, and thus concerned about the future of his sport. The facts he relates paints a grim picture:

- In the past 47 years, a million square miles of spring snowcover has disappeared from the Northern Hemisphere.

- Since 1970, the winter warming-rate has been triple the rate of the previous 75 years.

- The Alps are warming 2-3 times faster than the world-wide average.

- Potential Winter Olympic venues shrinks from 19 (now) to 6 by 2100.

and many other facts. Avid skiers like spring skiing. For March, the data (from Rutgers University Global Snow Lab) show a decline in area covered by snow.

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Rime, freezing fog, and crystalline spider webs

January 22nd, 2013

The Pacific Northwest has been foggy a lot lately, but the fog droplets have been subzero, or supercooled. When such fog droplets hit an object, they almost always freeze. The resulting frozen aggregate is called rime. Freezing fogs make rime.

The resulting rime formations may look like hoar frost, or even snow, from a distance. But close up the special features of rime become apparent.

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Frost Days and Ice Days: Declining Numbers over the Century

January 17th, 2013

David Easterling recently reported in BAMS** that the number of frost days per year is decreasing over the US. A frost day is a day in which the minimum temperature goes below the melting temperature of ice (32 F or 0 C). This doesn't sound good for a frost observer like me. Moreover, the largest decrease, a value of 2.6 fewer days per year per decade, is in my area, the Pacific Northwest. Will frost soon be a threatened species?

His analysis and presentation was based on 1948-1999 data, and moreover, he averaged over multi-state regions. I'm more interested in my local area, King County, Washington, and would like to see both the longer-term trend and variability. I quickly sent him an email, requesting more info. But I was impatient and sought the raw data myself. Following the info in his article, I found the data online***. An added plus: at many stations, the data now goes back another 50 years. With a few simple commands typed into an Excel spreadsheet, I determined the longer-term trend for a station near the University of Washington stadium in Seattle (the only such site in Seattle).

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